Linear actuation is very important in fields ranging from transportation, to controls, robotics and weapon systems. When considering the broadest range of linear actuators available to a mechanical design engineer, it is apparent that hydraulics and pneumatics offer unparalleled stress (defined as maximum force divided by actuator cross sectional area) and strain (extension divided by initial actuator length) explaining their dominance in power machinery. However, these are typically noisy and require heavy and bulky pressure sources such as motor driven pumps, reservoirs, accumulators, manifolds, complex servo valves, cooling equipment and plumbing. In addition, the actuation frequency of hydraulics and pneumatics is limited by valve dynamics and mass flow properties of liquids and gases.
Simple solid state actuation devices such as magnetostrictors, piezoelectrics and electrostrictors offer outstanding force capability (stress) but their strain output is so limited that it becomes extremely difficult to realize macroscopic strains in practical devices. Even with state of the art high strain piezoelectric single crystal materials originally developed to boost the output of low frequency sonar transducers the maximum available strain is <0.1% at high frequency >10 Hz and ˜1% at very low frequencies <10 Hz. Practical real world strains to produce useful macro-scale work can therefore only be obtained with huge solid state devices or motion amplification mechanisms that introduce phase lag as well as mechanical energy losses that generally appear as backlash and hysteresis.
Solenoids and moving coil actuators meet a range of meso- and macro-scale actuation requirements where low output force (low stress) actuation is needed at practically useful strains greater than 10−3. However, the performance of these devices does not extend into higher stress capabilities. Solenoids typically exhibit inertially limited frequency response and asymmetric nonlinear force versus displacement necessitating the use of a spring for bidirectional motion. Moving coils only develop modest, non-linear forces over a limited displacement range. In addition, these devices produce highly non-uniform forces over their displacement ranges because their construction is constrained by the need for very high local magnetic fields.
To achieve high performance in conventional magnetic actuators it is necessary to use high magnetic permeability materials, precisely aligned magnetic elements and the smallest possible air gaps between them. Typical solenoids require a tight air gap and need a high permeability magnetic material in the magnetic circuit. Similarly, conventional moving coil devices require a magnetic circuit arrangement and narrow gaps to generate reasonable displacement of the moving element. In both cases, the output force versus distance traveled by the moving element in the devices exhibits a nonlinear force versus distance behavior and consequent limitation on the range of useful displacement they can achieve. Linear motors extend the displacement available from an electromagnetic device by effectively stretching out the circumference of a rotational stepper motor. While this extends the range of motion, this type of device also relies on close spacing of the permanent and electro magnetic elements to provide enough magnetic field intensity to drive the moving element.
A typical conventional moving coil device requires a structure to both support the coil and connect it to the load, but it has to move through the most intense portion of the field. Accordingly, if the coil support consumes space that would otherwise be occupied by coil windings or increases the size of gap in the magnetic circuit the capabilities of the device are compromised. While the empty spaces, or voids, in moving coils are not configured in precisely the same way as those in solenoids, in both cases they present significant limitations on device performance. The precise thickness of these gaps is critical to the force and axial displacement obtainable from both because high uniform field strength must be maintained constantly across the narrowest practical gap. The conventional solenoid features two dissimilar types of empty spaces that break the magnetic circuit in the active magnetic train: a fixed width annular motion clearance gap; and an axial separation of variable length. The former causes a fixed amount of permeability degradation irrespective of displacement between moving and stationary parts, while the permeability decrement caused by the latter becomes higher with increased displacement and hence greater separation between moving and stationary parts. The moving coil has two parallel fixed width annular motion clearance gaps interrupting the magnetic circuit through the active magnetic train. While the widths of these two gaps orthogonal to the motion axis are small and fixed, the vertical (axial) position of the coil with respect to the adjacent inward facing planes of the yoke surrounding the opening changes as the coil travels vertically axially to produce force and displacement. Thus the path carrying magnetic flux between the coil and the yoke lengthens as the coil moves further away from its rest (no current) position centered vertically between the faces of the yoke surrounding the opening. Accordingly, the effective size of the gap is close to invariant for a relatively short vertical length while the coil remains in the swept volume between said yoke faces but then increases with coil travel in much the same way as axial separation in a solenoid.
While the above shortcomings of solenoids and moving coils have to be recognized, their direct linear electrical actuation (DLEA) remains very attractive because it represents direct conversion of electric current to mechanical force and motion without intermediary mechanisms (like gears for rotating motors) or media (such as fluids and gases in the case of hydraulics and pneumatics). However, such prior art DLEA devices present a control challenge when accurate forces and positions are needed with minimal overshoot and correction.
To respond rapidly to a commanded set point of force, acceleration, velocity or position, the control electronics need to have a-priori data or equations describing the force vs. distance and force vs. drive current (or voltage) of the device. If either or both relationships are nonlinear and/or vary with frequency then complex compensation, or linearization, circuits must be employed to allow a commanded, or desired, output to be translated into a practical corrected electrical drive signal to the device. Alternatively, with a digital control system, the device must be comprehensively characterized and equations or data describing the nonlinearity preloaded into the control routine. Two further control difficulties arise with the solenoid. First, its intrinsic unidirectional force and hence return-spring mode of operation for reverse motion confers an entirely different force-distance profile and force-current (or voltage) profile in either direction. As described above, this must be addressed mechanically, compensated electronically or programmed-out digitally. Secondly, the ascending force versus current (or voltage) path and the ascending force versus distance path will differ from the corresponding reverse (or descending) paths giving rise to hysteresis (an effect that also varies with frequency) and hence ambiguity in determining the instantaneous state of the system. For example a displacement feedback sensor can be used to determine force in a well-characterized and calibrated system but this inferred force will be quite different depending on whether the device is in ascending or descending mode.
Electromechanical (EM) actuators have become ubiquitous in almost every field of engineering, transportation, production, and consumer products from magnetic tape drives, industrial process lines, and robotics to aircraft and missile flight controls. Even though the EM actuator field is very mature, well understood, and devices have been successfully developed to address a wide range of applications, in almost all cases the underlying technology is based on the simple principles, and hence the limitations, of traditional solenoids and moving coils. A case in point is the limited amount of throw, or displacement, which can be produced by either type of device because both rely on close proximity of ferromagnetic elements, permanent magnets and/or current-carrying coils to focus the magnetic field within a narrow region. For many applications this has led to the use of electric motors with a variety of ancillary mechanisms (e.g., ball-screws, reduction gears and planetary gears, etc.) to realize greater displacement. The alternative has been to achieve long travel linear motion by employing a linearized stepper motor configuration with extended lengths of extremely small and tightly spaced coils and/or magnets. Both options result in increased electrical and control complexity as well as greater size and weight. The alternatives utilizing ancillary gears or ball-screws also present a finite risk of jamming failure as compared to the neutral, non-catastrophic graceful failure modes that can be invoked with hydraulic actuation systems, which is critical for high-reliability applications such as aircraft flight controls.